1. Field of the Invention
The invention relates to the current protection of a distribution box, and more particularly the current protection of a primary electrical distribution box supplied by a generator and delivering electrical energy to at least one secondary distribution box.
2. Description of the Relevant Art
The distribution of electrical energy in an aircraft is generally arranged in a pyramidal hierarchy. First of all, the electrical power is produced mainly from engine-driven generators. This energy is then conveyed by means of cables to primary distribution boxes. These primary distribution boxes then send this electrical energy directly to loads or to secondary distribution boxes.
In general, each of the primary or secondary boxes include the distribution components among which some, like the line contactor, have the function of switching this electrical energy to a distribution busbar included in the distribution box, a primary distribution busbar in a primary box and a secondary distribution busbar in a secondary box. They also include distribution components which have the function of switching electrical power to electrical loads on board, or even distribution components having the function of switching electrical energy to secondary distribution subnetworks that further redistribute the electrical energy to loads of lesser power.
Other distribution components have the function of protecting the network in case of a fault, such as a short-circuit, for example.
Systematically, each cable coming out of the distribution box for routing electrical energy is protected by a protection component such as, for example, a circuit breaker, a fuse, or a contactor breaker. These components are all based on a trip from a current overload, and are therefore activated according to a trip time inversely proportional to the overload current.
The trip time of these protections is defined so that in case of a fault, the protection element situated directly upstream cuts out on its own without any other protection component tripping.
Otherwise, where a protection element of higher rank, i.e. farther upstream, trips, the faulty line would be isolated, as well as other healthy lines causing problems. This is the principle of selectivity. Thus, the protections of the secondary network trip faster than those farther upstream, of the primary network.
The protection farthest upstream of the electricity network is that which controls the generator and which controls the line contactor, the switching element farthest upstream. It is also based on a current measurement in order to identify an overload. It can be used to isolate the fault if no other downstream protection could do so, thus proving that the fault is situated downstream at the level of the primary network. For this last protection to trip, it must, however, wait for enough time for all downstream protections to be able to trip if the fault is situated at their level. This time can take up to 5 s, which is long enough to cause significant damage.
By creating a hierarchical protection such as that above, the generator protection can operate only after all the others, and is therefore the longest to trip. Consequently, a fault directly downstream of the generator protection, but upstream of all the other protections, may cause damage before being isolated given the trip time of this last generator protection. This damage may then be harmful to the operation of the electrical distribution system but especially to the continued mission of the aircraft.
Other types of protection are also known for obviating the need for this hierarchical principle, this is differential protection. This protection consists in making a measurement upstream and a measurement downstream of a distribution line. In the absence of a fault, the current measured upstream and downstream of the distribution line by the two sensors is identical. On the other hand, in case of a fault, the current measured by the two sensors differs since a part of the current is then directed to the structure at the location of the short circuit. This principle is generally used for protecting generator cables. This principle of protection is also sometimes used for cable protections called inter-busbar protections between two primary busbars of the same primary box or in two primary distribution boxes.
This protection is, on the other hand, very difficult to implement for protecting a distribution box from an internal short circuit since a current comparison then has to be made between a supply line, that coming from the generator, and several distribution lines for the supply of loads and of the secondary network. Given the number of lines to be measured and the disparity, however small, of the sensors used, there is a high risk of nuisance tripping of the protection element.